The present invention relates to the field of engine brakes of a vehicle. Especially for a vehicle provided with a combustion engine having cylinders with cylinder valves an exhaust pressure governor (EPG) regulating the air flow out of the cylinders and an intake air throttle valve (ITV) regulating the air flow into the cylinders.
Engine brakes which comprise a compression brake and an exhaust pressure governor (EPG) are known. The compression brake closes the cylinders valves, such that the air therein is compressed, whereby a brake torque is created. Normally, the compression brake is controlled by an on/off valve.
The EPG controls the pressure downstream of the cylinders, wherein a closing of the EPG usually leads to a higher exhaust manifold pressure, thereby a higher engine brake torque. The EPG is usually controlled with a closed-loop control with the exhaust pressure as feedback signal.
The total engine brake torque is a combination of the brake torque contribution from the compression brake and the EPG.
The inputs to a controller of the compression brake are the demanded exhaust pressure and the actual exhaust pressure. The output from the controller of the compression brake is a control signal that controls the movement of the EPG. During engine braking, the exhaust pressure is proportional to the engine brake torque and is therefore used to indirectly control the engine brake torque.
For some engines, especially turbo compound engines, and at some engine speeds, it is not possible to control the brake torque contribution from the compression brake between a zero brake torque contribution from the compression brake when it is deactivated and the maximum brake torque contribution from the compression brake that can be achieved with the compression brake activated. Due to the fact that the compression brake is activated by an ON/Off valve, continuous control of engine brake torque is not possible between the maximum torque that can be reached with only the EPG and the torque that is reached with the compression brake only.
Hence, during some conditions, the engine brake torque regulation cannot be regulated indefinitely or in small discrete steps, instead just in an on/off mode, due to the on/off regulation of the compression brake.
There is thus a need for an improved regulation of a vehicles engine brake, which removes the above mentioned disadvantage.
It is desirable to provide an inventive method to control an engine brake of a vehicle, wherein said method facilitates better control possibilities of the engine brake.
The inventive method to control an engine brake of a vehicle is adapted for a vehicle provided with a combustion engine having;
cylinders allowing a compression braking therewith,
an exhaust pressure governor (EPG) regulating the air flow out of the cylinders,
an intake air throttle valve (ITV) regulating the air flow into the cylinders, and
pressure sensing means for sensing a pressure downstream of the cylinders.
The engine brake of said vehicle is adapted to be regulated in two different engine brake modes;
a first engine brake mode, in which the air flow through the EPG is regulated by a closed loop control using said pressure downstream of the cylinders and the ITV is regulated in a feed forward control dependent of the engine speed and a demanded brake torque, and
a second engine brake mode, in which the EPG is regulated in a feed forward control dependent of the engine speed (S) and the demanded brake torque (T), and the ITV regulates the braking torque by a closed loop control using said pressure downstream of the cylinders.
When the ITV is regulated such that the intake air mass flow in the cylinders of the engine is decreased, the braking torque contribution from the compression brake is decreased. An infinite or discrete regulation of the compression brake can thereby be achieved.
Because both the engine brake modes regulates the braking torque with a closed loop control against the pressure downstream of the cylinders a smooth transition between the two different engine brake modes is facilitated.
In the first regulation mode, the EPG is regulated dependent of the sensed pressure downstream of the cylinders, wherein the ITV is regulated in a feed forward control dependent of the engine speed and a demanded brake torque. The position of the ITV is recalled from a two-dimensional map or a list having the engine speed and the demanded brake torque as input signals. The map or list is preferably predetermined and stored in the engine brake controller.
In the second regulation mode, the EPG is regulated in a feed forward control dependent of the engine speed and demanded brake torque. The ITV regulates the braking torque in direct dependency of the sensed pressure downstream of the cylinders. The position of the EPG is recalled from a two-dimensional map or a list having the engine speed and the demanded brake torque as input signals. As before, the map or list is preferably predetermined and stored in the engine brake controller. The second regulation mode is used if the EPG is already completely open and less torque/exhaust pressure is requested, wherein this regulation has to be done with the ITV, whereby the engine brake can be more exact regulated over a greater torque span.
A determination of which of the first and the second engine braking mode that should be used is dependent on a demanded braking torque and an actual engine speed, thereby can always the optimal braking torque regulation be used for all situations of operation of the engine.
It is preferred that the sensing means for sensing a pressure downstream of the cylinders, senses the exhaust manifold pressure from the cylinders. Existing pressure sensors for sensing the exhaust manifold pressure from the cylinders can thereby be used without any additional cost.
It is preferred that the second braking mode is used when a demanded brake torque is below a brake torque threshold value, or an actual engine speed is above an engine speed threshold value. At high engine speeds, an activation of the compression brake gives very high braking torque, where by limit values of the engine can be exceeded, i.e. exhaust temperature, pressure differences over exhaust valves, etc, by controlling the engine brake in the second mode this can be avoided, in that the brake torque is reduced using the ITV.
The second brake mode is also preferred at lower engine speeds and a low brake torque demands.
It is further preferred that said first brake mode is used when a demanded engine brake torque is above an engine brake torque threshold value and an actual engine speed is below an engine speed threshold value. The highest brake torque is achieved when both the EPG and the compression brake is controlled to deliver a maximum brake torque.
It is further preferred that a switching from said second braking mode to said first braking mode is done, when the demanded braking torque increases above an engine braking torque threshold value and the engine speed is below an engine speed threshold value.
It is further preferred that a switching from said first braking mode to said second braking mode is done, when the demanded braking torque is decreasing below an engine torque threshold value, or when the actual engine speed increases above an engine speed threshold value, or when the EPG is completely open and the demanded exhaust manifold pressure is lower than an actual exhaust manifold pressure, or when an EPG actuator failure occurs. An optimal regulation of the braking torque is thereby achieved for all operation conditions of the engine.
It is further preferred that said engine is equipped with a charge air cooler bypass valve (CAC-valve), whereby during engine braking said CAC-valve can be controlled to increase or decrease said exhaust manifold pressure. The CAC-valve can be regulated in the same exact manner, and is suitable to regulate against the pressure downstream of the cylinders, e.g. the exhaust manifold pressure. Thereby can an engine brake controller choose to regulate the air mass flow into the cylinders with either the CAC-valve or the ITV. The temperature of the exhaust gas can thereby be regulated, which is important in order to achieve high enough temperatures for the exhaust gas after treatment system.
It is further preferred that said engine torque threshold value comprises a first and a second engine torque threshold value, wherein said first engine torque threshold value is lower than said second engine torque threshold value, and said engine speed threshold value comprises a first and a second engine speed threshold value, wherein said first engine speed threshold value is lower than said second engine speed threshold value, wherein said first threshold values are used when the reference value increases and the second threshold value are used when the reference value decreases. By using a hysteresis function as described above, unnecessary switching between the two modes of regulation are avoided in the bounder areas.
It is further preferred that said first engine torque threshold value is dependent of the engine speed.
The invention also relates to an engine brake system for a vehicle, where a control unit is arranged to perform said method steps.